The invention relates to a method of automatic gear changing in a mechanical stepped gearbox, and particularly to adjustment of the torque delivered by the engine during a gear change.
Automatic gear changing in mechanical stepped gearboxes requires the torque delivered from the engine to be adjusted in order to reduce the torque transmitted at the point of contact of the relevant gears. At the time of gear disengagement it is desirable for there to be a torque-free state at the contact point between the gears in the gearbox, but since it is not practically possible to measure the torque at the gear contact point the torque adjustment has to be achieved indirectly by adjusting the torque delivered from the engine. SE,C,9401653-2 refers to a solution whereby gear changing takes place without the disc clutch being released. In this case a gear change is preceded by the engine torque being adjusted towards a zero-torque level which is intended to lighten the load on the gearbox during the gear change. At this stage the torque with which the engine is modulated is calculated on the basis of available data concerning the engine, its moment of inertia, its internal torque and the like. The modulated torque may also depend on whether any power take-offs are operating or not.
Disadvantages of this method are that there is no direct measurement of the torque and that the modulation is based on the expected behavior of the engine. It is therefore not known whether the modulated torque is correct or not. The manufacture of engines and the various driveline components may involve tolerance differences which result in each individual vehicle having different characteristics. Even if the behavior of an engine can be predicted correctly when it is new, its characteristics will change over time and with engine wear. This means that the engine torque modulated over time is probably not the same as when the engine was new. This in its turn means that gear changing may become difficult and/or that it is unacceptably delayed in certain circumstances. Oscillations in the control system may result in a very long scaling-in time being required for being able to determine correctly the fuel quantity which gives constant synchronous speed.
SE,C,9600454-4 refers to a solution which aims at improving that described above. To this end, a measurement is carried out at each gear change to ascertain whether the modulated zero-torque was correct. If it was not, the incorrect torque detected is used to correct the original zero-torque value so that before a subsequent gear disengagement the engine is modulated with a more correct zero-torque value. During the measurement, the amount of change in the engine speed with the fuel supply unchanged is registered for a certain measuring period after the gear disengagement and with the gearbox in neutral position. Knowing the length of the measuring period and the engine""s moment of inertia provides a measure of the corresponding torque deviation, i.e. the amount of torque by which the zero-torque value applied needs adjusting so that gear disengagement can take place in a torque-free state of the gearbox. The corrected torque level is used as a basis for a calculation which determines when correct gear disengagement will take place.
A disadvantage of this method is that the measurement takes a relatively long time, approximately 0.5 second or longer, thereby adversely affecting total gearchange time. Particularly when driving on steep uphill runs with heavy loads it is desirable to have the shortest possible gearchange time so that the vehicle will not lose too much velocity during gear changes.
Experience also shows that for technical reasons of engine design it has proved difficult in the control of new engine systems to achieve a simple match between the fuel quantity supplied and the torque obtained at the low loads which prevail at the time of gear changes. A given quantity of fuel does not always result in the intended engine torque, so setting the desired zero torque becomes unreliable, with consequently problematic gear changing.
Zero torque not prevailing at the time of gear disengagement is primarily perceived as a disturbance to comfort whereby an oscillation is initiated in the driveline. This oscillation then causes a time delay before the next gear can be engaged. The engine""s zero torque depends mainly on its internal friction and may therefore vary with such factors as wear, temperature and engine speed. The determination and correction of the zero torque has therefore to be carried out at regular intervals so that there is an applicable value of it for the whole life of the vehicle.
One object of the invention is to make it possible more reliably than previously to ensure that the torque transmitted in the gearbox will be nil at the time of gear changes. A further object is to be able to shorten the time between disengaging one gear and engaging the next.
The objects indicated above are achieved by the invention having an effective engine torque level adjustment for affecting the gear change. A control system reduces the torque transmitted in the gear box at the time of the gear change by adjusting the torque delivered by the engine to a predetermined zero torque level. Disengagement of a gear to the neutral position takes place at that zero torque level. The zero torque level is corrected between gear changes if there is a difference between the predetermined zero torque level and the actual zero torque level that is sensed. For this purpose, the speed of the gear box output shaft is detected. Then there is a speed change during a predetermined short period of time immediately after disengagement of the operative gear to the neutral position which adjusts the predetermined zero torque level as a function of the speed change. In particular, a speed change in the form of an increase raises the zero torque level while a speed change in the form of a speed reduction lowers the zero torque level. The predetermined short period of time amounts to not more than 50 ms and the adaptation of the engine speed for engaging another gear, in particular, a different gear commences at the time of disengagement of the previous gear. The solution adopted according to the invention results in the engine torque adjustment being possible with greater precision than previously. It also makes it possible to shorten the time needed between disengaging one gear and engaging the next.
The method according to the invention makes it possible to achieve more reliable and quicker gear changes, particularly when driving with heavy loads on uphill runs.
Other features distinguishing the invention are indicated by the following description of an embodiment with reference to the attached drawings.